IN ANY SINGLE FORMATION. 307 



during some part of the glacial period shall have been up- 

 raised, organic remains will probably first appear and disap- 

 pear at different levels, owing to the migrations of species 

 and to geographical changes. And in the distant future, a 

 geologist, examining these beds, would be tempted to con- 

 clude that the average duration of life of the embedded 

 fossils had been less than that of the glacial period, instead 

 of having been really far greater, that is, extending from 

 before the glacial epoch to the present day. 



In order to get a perfect gradation between two forms 

 in the upper and lower parts of the same formation, the de- 

 posit must have gone on continuously accumulating during a 

 long period, sufficient for the slow process of modification ; 

 hence, the deposit must be a very thick one ; and the spe- 

 cies undergoing change must have lived in the same district 

 throughout the whole time. But we have seen that a thick 

 formation, fossiliferous throughout its entire thickness, can 

 accumulate only during a period of subsidence ; and to keep 

 the depth approximately the same, which is necessary that 

 the same marine species may live on the same space, the 

 supply of sediment must nearly counterbalance the amount 

 of subsidence. But this same movement of subsidence will 

 tend to submerge the area whence the sediment is derived, 

 and thus diminish the supply, while the downward move- 

 ment continues. In fact, this nearly exact balancing be- 

 tween the supply of sediment and the amount of subsidence 

 is probably a rare contingency ; for it has been observed by 

 more than one palaeontologist that very thick deposits are 

 usually barren of organic remains, except near their upper 

 or lower limits. 



It would seem that each separate formation, like the whole 

 pile of formation in any country, has generally been inter- 

 mittent in its accumulation. When we see, as is so often 

 the case, a formation composed of beds of widely different 

 mineralogical composition, we may reasonably suspect that 

 the process of deposition has been more or less interrupted. 

 Nor will the closest inspection of a formation give us any 

 idea of the length of time which its deposition may have 

 consumed. Many instances could be given of beds, only a 

 few feet in thickness, representing formations which are 

 elsewhere thousands of feet in thickness, and which must 

 have required an enormous period for their accumulation ; 

 yet no one ignorant of this fact would have even suspected 

 the vast lapse of time represented by the thinner formation. 



